Documentation and References
Features references and tutorials covering HTML, CSS and the DOM.
Provides dozens of tutorials, references and examples in all categories of web building.
World Wide Web Consortium (W3C) Sites
The full specification for CSS Level 1 which mostly concerns colors, fonts and spacing.
The full specification for CSS Level 2 which adds support for layout. This revision corrects some errors and omissions in the previous specification.
Cascading Style Sheets
Contains dozens of links to references, tutorials and examples.
The full specification for HTML 4.01 covering the basics along with all tags and attributes.
With a few minor changes, HTML 4.01 pages can be made into XML-compliant documents. This specification describes those changes.
Some helpful online tools that let you check your pages for standards compliance. Good for finding errors when your pages don't display as expected.
General Information and Code Samples
An excellent collection of CSS examples, articles and tutorials.
css Zen Garden
One HTML page, dozens of style sheets. An excellent demonstration of CSS's versatility.
Glish.com: CSS Layout Techniques
Offers several examples showing how CSS can be used instead of tables for creating page layouts. Includes details on how it's done.
Eric Meyer offers expert advice for using CSS and other web related technologies. You'll find several excellent and inspiring examples on his site.
Position is Everything
Demonstrates, explains and suggests workarounds for common CSS-related bugs in various browsers.
thenoodleincident.com discusses browser compatibility issues and provides some helpful layout samples.
"Everything you ever wanted to know about Style"
The title pretty much sums it up. For a commercial software and consulting company, they offer a good deal of useful, free information.
The Open Directory Project's CSS listings.
From the best search engine on the Web.